System and method for precharging and discharging a high power ultracapacitor pack

ABSTRACT

This invention is a system and a method that uses the braking resistor, commonly used and available in electrically or hybrid-electrically propelled vehicles, to limit the precharge current during the startup of a high power ultracapacitor pack energy storage device and/or safely and rapidly discharge an ultracapacitor pack for maintenance work or storage to lengthen the life of the individual ultracapacitor cells and, correspondingly, the whole pack. The use of the braking resistor for precharging an ultracapacitor energy storage pack is an effective and less expensive method compared to other methods such as a separate DC-to-DC converter. This method includes the control logic sequence to activate and deactivate switching devices that perform the connections for the charging and discharging current paths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/628,030 filed Nov. 15, 2004 under 35 U.S.C. 119(e).

FIELD OF THE INVENTION

The field of the invention relates to systems and methods for thestartup initial charging and the shutdown discharging of a high-voltage,high-power ultracapacitor energy storage pack composed of a large numberof serially connected individual low-voltage ultracapacitor cells thatstore an electrical charge.

BACKGROUND OF THE INVENTION

The use of ultracapacitor packs for high-voltage, high-power energystorage applications is well known (See, for example, U.S. Pat. Nos.6,844,704 and 6,714,391). However, the high current, low resistancecharacteristics of ultracapacitors present a problem during the startup(charging) phase for a completely discharged pack and the shutdown(discharging) phase of a charged pack.

Due to the low resistance of the ultracapacitors, it is usually notpossible to connect the pack to a high voltage source by simply closinga high power contactor relay switch. If this is done, the initialconnection of an ultracapacitor pack to a charging circuit looks like adirect short to the charging circuit and the resulting high currentinrush into the ultracapacitor pack from the charging circuit can easilydamage the charging circuit. One method of initially charging acompletely discharged pack uses a high-power DC/DC converter so thatvoltage is increased slowly to limit the current flow in the circuit.Two problems with using the high-power DC/DC converter is that thiscomponent is very expensive, adding significant cost to the overallsystem, and is an additional component, adding complexity to the system.Furthermore, if the DC/DC converter remains in the circuit during normaloperation to minimize the high voltage drop as the ultracapacitor packdischarges, the energy storage and supply cycle experiences the reducedefficiency of the two way energy path through the DC/DC converter.

At the end of an operation period, typically at the end of the day, itis desirable to discharge the ultracapacitor pack for safety, cellequalization, and increased cell life. Reducing the stand-by voltageacross each cell is a means to increase the cell and pack lifetime. Apassive balancing network consisting of a resistor in parallel with eachcell may discharge an ultracapacitor pack but it typically requireshours for the voltage to drop to a minimum level. An active circuit tobalance and/or discharge each cell may also drop the pack voltage, butit adds more cost and complexity to the ultracapacitor pack.

SUMMARY OF THE INVENTION

The present invention involves a method for precharging and/ordischarging an ultracapacitor pack where a braking resistor, a commoncomponent used to dissipate power from an electro-magnetic brakingregeneration system in hybrid electric vehicles, is connected in serieswith the ultracapacitor pack during startup to limit the prechargecurrent into the ultracapacitor pack, eliminating the need for a veryexpensive, high-power DC/DC converter. Similarly, during shutdown thebraking resistor is connected across the pack to safely and rapidlydischarge the pack to a minimum stand-by level. This is an effective,low-cost and safe method of precharging and/or discharging anultracapacitor pack. The invention also utilizes other componentscommonly used on hybrid-electric vehicles: an engine/generator, aninverter, and various high power switching relays called contactors.

In a typical hybrid-electric vehicle application an ultracapacitor packis charged from either the engine/generator, or the traction motoroperating in the braking regeneration mode.

The generator can supply all the power necessary to quickly charge theultracapacitor pack, but the generator is always producing some minimumvoltage much higher than zero (e.g. 200 volts) due to its permanentmagnet design and because it is connected to an engine, which is runningat some minimum speed (idle). In general, the voltage is too high toallow a direct connection to the ultracapacitors.

An ultracapacitor pack is excellent for storing the high power brakingregeneration energy where the traction motor operates as a generator toapply a drag on the driveline and decelerate the vehicle. Brakingregeneration also reduces wear and maintenance on the mechanical brakingsystem. However, when there is no energy storage or the energy storageis charged to capacity the electromagnetic braking regeneration systemdissipates power through a braking resistor and/or by using thegenerator as a motor to spin the engine with the fuel supply cut off. Atypical braking resistor for a heavy-duty vehicle is liquid cooled andhas a 60 kW power rating. One or more of these resistors may be used ona heavy-duty vehicle.

Spinning a non-fueled engine dissipates power as it converts thespinning energy into heat by working against the compression of theengine pistons and cylinders and transfers energy to the accessoriesthrough the belt and/or gear power-take-offs on the engine. A typicalheavy duty engine that is spun by a generator can absorb 30 kW of power.Spinning the engine in this way may also use the engine water pump tocontinue circulating cooling fluid through the braking resistor.

The inventors have recognized that when a hybrid-electric vehicle hasboth components, an ultracapacitor pack for energy storage and a brakingresistor for excess power dissipation, the braking resistor can beconnected in series with the ultracapacitor pack during startup to limitthe precharge current into the ultracapacitor pack and/or connectedacross the ultracapacitor pack during shutdown to safely and rapidlydischarge the ultracapacitor pack to a standby level. Contactors or highcurrent IGBT (Insulated Gate Bipolar Transistor) solid state switchesare used to implement the connections.

The braking resistor limits the in-rush current, and the high powerrating of the braking resistor allows for rapid initial precharging ofthe ultracapacitor pack without overloading the charging circuit.Another consideration is that if the uncharged ultracapacitor pack wereconnected directly to the generator during engine start up, the enginestarter does not have enough torque to turn the engine against thetorque of a generator that sees a shorted DC output bus.

Once the engine/generator has started with the braking resistorconnected in series and the ultracapacitor pack precharging has begun,this method may also include some voltage regulation of the generatoroutput. For example, as an option to quicken the precharge process, thegenerator output voltage can be increased to compensate for the voltagedrop across the resistor. When the ultracapacitor pack is sufficientlycharged to match the generator output voltage the braking resistor isswitched out of the circuit and the generator is connected via thegenerator control inverter directly to the ultracapacitor pack.

During the normal operation of the generator and ultracapacitor packenergy storage no further precharging is required for voltage matching.When the ultracapacitor pack is charged the engine may be turned off forshort time periods while the ultracapacitor pack supplies all the powerdemands from the high voltage bus. As the ultracapacitor pack reaches aminimum level of energy storage, the pack supplies the power to thegenerator to spin the engine for a restart without using the low voltageengine starter. Upon restarting the engine the generator again suppliespower to the high voltage bus for the vehicle power requirements and torecharge the ultracapacitor pack to a minimum operating level. Theprecharge process is not repeated again until the vehicle starts up froma discharged energy storage pack.

An ultracapacitor pack may have an active or passive voltage balancingcircuit for the individual capacitors and the pack may be designed toself-discharge overnight. A precharge process is required any time thepack voltage drops below the minimum generator voltage.

In another aspect of the invention, the ultracap pack is prechargedthrough braking regeneration (i.e., the ultracap pack is prechargedthrough the braking resistors using the energy from the drive/tractionmotors when they act like generators in the process of doing electricbraking). In this case braking regeneration energy is used instead ofenergy from the engine/generator. This variation has the same switchconnections to the high voltage DC bus as the embodiment(s) describedabove, but the source energy is from the drive/traction motors. In thisembodiment the switching is performed through Siemens DUO-Inverterswitches shown and described herein. Different embodiments may use otherinverters and switches. This embodiment may be advantageous, forexample, if the engine would not start with the starter, but thehybrid-electric vehicle could be set rolling (e.g., downhill), then thebraking regeneration could store enough energy in the ultracap packs toget the engine started from the generator as is normally done duringoperation after startup.

In a further aspect of the invention, the ultracap pack is immediatelydischarged through the braking resistor for service and maintenancesafety, and as an end-of-the-day turn-off for quicker equalization inthe ultracap pack without having to depend on the much slower leak-downdischarging current across the passive parallel resistors within theultracap pack. The immediate discharge may be accomplished throughSiemens DUO-Inverter switches shown and described herein. Differentembodiments may use other inverters and switches.

In another aspect of the invention the inverter IGBT switches are set todischarge the ultracap pack through the generator by spinning the enginewith the fuel cut off. Thus, the ultracapacitor pack may be dischargeddown to the minimum operating voltage of the generator. In this way thegenerator can be used in place of or in addition to the braking resistorto rapidly discharge the ultracapacitor pack. There may be an advantagein using the two methods together because the spinning engine willcontinue to pump engine coolant through the cooling circuit thattypically includes the liquid cooled braking resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and togetherwith the description, serve to explain the principles of this invention.

FIG. 1 is a block diagram depicting an embodiment of a hybrid-electricvehicle drive system precharging an ultracapacitor energy storage packthrough a high-power braking resistor.

FIGS. 2A–2E are a logic flow diagram of an exemplary control sequencefor the hybrid-electric vehicle drive system illustrated in FIG. 1 thatincorporates the braking resistor to precharge an ultracapacitor pack.The control sequence includes four functions or processes: 1) Start UpInitialization and Ultracapacitor Precharging (FIG. 2A, 2B), 2) RunModes: Acceleration (FIG. 2C) and 3) Deceleration (FIG. 2D), and 4)Safety Shutdown Discharge (FIG. 2E).

FIG. 3 is a circuit schematic diagram that shows an embodiment of theinvention that uses IGBT switching in a standard 3-phase inverter powerstage.

FIG. 4 is a circuit schematic diagram that shows an embodiment of theinvention that uses IGBT switching in a standard 8-phase SiemensDuo-inverter where six phases control two 3-phase motors and the tworemaining phases perform the switching for the braking resistor andultracapacitor pack.

FIG. 5 has four different circuit schematic diagrams; 5A, 5B, 5C, and5D; that show four alternative embodiments of the invention using IGBTswitching with two phases of any inverter.

FIG. 6 has two different circuit schematic diagrams; 6A, and 6B; thatshow two alternative embodiments of the invention using IGBT switchingwith four phases of any inverter.

FIG. 7 has two different circuit schematic diagrams; 7A, and 7B; thatshow two more alternative embodiments of the invention using IGBT withfour phases of any inverter.

FIG. 8 has two different circuit schematic diagrams; 8A, and 8B; thatshow two more alternative embodiments of the invention using IGBTswitching with four phases of any inverter. A preferred embodiment isshown in the schematic diagram of FIG. 8A that uses the remaining twophases in each of two single 8-phase Siemens Duo-Inverters

FIG. 9 is an electrical circuit schematic diagram that depicts thepreferred embodiment of FIG. 8A as configured in a hybrid-electricvehicle drive system with two Siemens Duo-Inverters that also control agenerator, two drive motors, and AC auxiliary power for the vehicle'selectric accessories consisting of the hydraulic pump, air compressor,and air conditioning compressor.

FIGS. 10A and 10B are a combined electrical circuit schematic andalgorithm flow chart for an exemplary method of precharging/dischargingan ultracapacitor energy storage pack through a high-power brakingresistor.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, the block diagram depicts an embodiment of ahybrid-electric drive system 100 with an ultracapacitor pack 110 forenergy storage and a braking resistor 120 for extra deceleration powerdissipation. For normal operation switch 1 is closed and switch 2 is inthe A position, connecting the ultracapacitor pack 110 to the power bus130, and switch 3 is closed whenever it is desired to use the brakingresistor 120. With switch 1 open, switch 3 open, and switch 2 in the Bposition the braking resistor 120 is connected in series with theultracapacitor pack 110. Thus, the ultracapacitor pack 110 can becharged from the power bus 130 through the braking resistor 120. Switch2 and the connection to the braking resistor 120 are the only additionsto the hybrid-electric drive connection for allowing the hybrid-electricdrive connection to be used for precharging the ultracapacitor pack 110through the high-power braking resistor 120. This saves the expense of aseparate precharge circuit by using the already present braking resistor120 that has the power handling capacity to limit the initial chargingcurrent to the ultracap pack 110.

At the conclusion of the precharge function switch 2 is switched to theA position for normal operation. Because the precharge function happenstypically only once a day, mechanical high power contactors could beused to implement the function of switch 2. However, the preferredembodiment as described below shows how the Insulated Gate BipolarTransistors (IGBT's) of an inverter/controller are used to implement allthe switching functions.

With reference to FIGS. 2A–2E, the exemplary control logic flow ofhybrid-electric drive system 100 that incorporates this inventionconsists of four functions or processes: 1) Start Up Initialization 140and Ultracapacitor Precharging 280 (FIG. 2A, 2B), 2) Run Modes 150:Acceleration 160 (FIG. 2C) and 3) Deceleration 170 (FIG. 2D), and 4)Safety Shutdown Discharge 180 (FIG. 2E). For the purposes of thisdescription braking and/or deceleration occurs any time that there is noacceleration and can be activated by releasing the accelerator pedaland/or applying the brake pedal.

With reference to FIG. 2A, at Start Up 140, the system 100 isinitialized by checking for the correct setting of all the switches. Atstep 170, a determination is made as to whether generator 180 isconnected to the power bus 130. If yes, control is passed on to step190, where the generator 180 is disconnected and then control is passedon to step 200. If no, control is passed on to step 200. At step 200, adetermination is made as to whether the ultracap pack 110 is connectedto the power bus 130. If yes, control is passed on to step 210, wherethe ultracap pack 110 is disconnected and then control is passed on tostep 220. If no, control is passed on to step 220. At step 220, adetermination is made as to whether the braking resistor 120 isconnected to the power bus 130. If yes, control is passed on to step230, where the braking resistor 120 is disconnected and then control ispassed on to step 240. If no, control is passed on to step 240. At step240, engine 250 starts via a standard 12 or 24VDC starter and providesthe kinetic power to turn the generator 180, and the generator 180 isconnected to the power bus 130. At step 270, the generator controlinverter 260 charges the high power bus 130 up to an operating voltagelevel, e.g., 300 to 500VDC.

With reference to FIG. 2B, to begin precharge 280, at step 290, thebraking resistor 120 is connected in series with the ultracapacitor pack110 and connected to the high power bus 130. Next, at step 295, thegenerator control inverter 260 controls the voltage of the high powerbus 130 to control the charging current passing through the brakingresistor 120 to the ultracap pack 110. Thus, the charging current isadjusted for a safe fast charge time. At step 330, a determination ismade as to whether the ultracap 110 voltage matches the voltage of thehigh power bus 130. If no, the control is passed back to step 295 tocontinue controlling the charging current by adjusting the generator 180output voltage. If yes, control is passed on to step 340, where thebraking resistor 120 is safely disconnected from the ultracap pack 110,the ultracap pack 110 is connected to the high power bus 130. At theoptional step 345 the generator 180 is disconnected from the power bus130 and the engine 250 is turned off.

Otherwise, with reference to FIG. 2C, after precharge 180, the vehiclecontrol continues into a normal running mode 150 at step 350 where thegenerator 180 and the ultracapacitor pack 110 are both connected to thepower bus 130. As first determined at step 370 the control system willbe in either an acceleration mode 160 (FIG. 2C) or deceleration mode 170(FIG. 2D) for the rest of the day until the vehicle is turned off andthe ultracap pack 110 is discharged.

The flow chart for the acceleration mode 160 illustrated in FIG. 2C willbe described after generally describing the acceleration mode 160.During acceleration 160 the vehicle accelerates on energy storage powerfrom ultracap 110 and power from engine/generator 250/180 until theultracap 110 stored energy level drops below a minimum threshold, wherethe ultracap 110 is disconnected from the power bus and theengine/generator 250/180 power alone accelerates the vehicle until thecontrol switches to the deceleration mode 170.

With reference to the flow chart illustrated in FIG. 2C, at step 370, adetermination is made as to whether the vehicle is acceleration. If no,control passes on to the deceleration mode 170 (FIG. 2D). If yes, thevehicle is in acceleration mode 160 and control is passed on to step380, where the vehicle control accelerates the vehicle using both energystorage power from the ultracap 110 and power from the engine/generator250/180. At step 390, a determination is made as to whether the ultracap110 energy level is below a minimum threshold (as determined by aminimum voltage o the power bus 130). If no, control is passed back tostep 370, where a determination is made as to whether the vehiclecontrol is still in acceleration mode 160. If no, control passes on tothe deceleration mode 170 (FIG. 2D). If yes and acceleration is stilldesired, control passes on to step 380 and again passes control to step390. At step 390, if the power bus voltage is below the minimumindication for the ultracap 110 storage level, control passes on to step410, where the ultracap pack 110 is disconnected from the power bus 130and the vehicle continues to accelerate on power from engine/generator250/180. At step 400, a determination is made as to whether accelerationis still desired. If no, control is passed on to deceleration mode 170.If yes, vehicle control continues to accelerate the vehicle at step 360and control passes back to step 400 where again a determination is madeas to whether acceleration is still desired. If no, control is passed onto deceleration mode 170 at step 500 (FIG. 2D).

The flow chart for the deceleration mode 170 illustrated in FIG. 2D willbe described after generally describing the deceleration mode 170. Oncedeceleration control is sensed the system inverter/controllers 260switch the traction motors 412 into a regeneration mode and match thevoltage of the power bus 130 to the voltage of the ultracapacitor pack110 to connect the ultracapacitor pack 110 if it is not alreadyconnected. The ultracapacitor pack 110 immediately starts receiving theregeneration charge. The charge current into the ultracap pack ismonitored for a maximum current threshold, e.g. 300 Amps. When theinverter controller 260 can no longer maintain the maximum currentlimit, or the ultracap pack 110 energy storage is filled to capacity,the control switching connects the braking resistors 120 to the powerbus to dissipate energy that cannot be stored. In practice, theengine/generator controller 260, can also absorb some energy by usingthe generator 180 to spin the engine 250 with the fuel supply turnedoff. When more braking is needed beyond the regeneration capacity of thedrive system, the mechanical brakes are also applied to decelerate thevehicle.

With reference to the flow chart illustrated in FIG. 2D, upondeceleration, at step 500, as the deceleration control and brakingregeneration are engaged, if the ultracap pack 110 is not alreadyconnected, the voltage of the power bus 130 is matched to the voltage ofthe ultracap pack 110, and the ultracap pack 110 is connected to thepower bus 130. At step 510, control passes to an algorithm that makes adetermination on how to best charge the ultracap pack 110 from both thebraking regeneration motors 412 and the engine/generator 250/180 to havea full energy storage by the time the vehicle stops to be ready for thenext start up acceleration. At step 520, a determination is made as towhether to continue in the deceleration mode 170 or switch to theacceleration mode 160. If the vehicle control indicates going to theacceleration mode 160, control passes to step 560 where, if connected,the braking resistor 120 is disconnected from the power bus 130. If thecontrol continues with the deceleration 170, control passes to step 530where a determination is made if the ultracap pack 110 is at its maximumcurrent threshold (e.g. 300 Amps) or if the ultracap pack 110 is chargedto capacity. If yes, control is passed on to step 540, where, If notalready connected, the braking resistor 120 is connected to the powerbus 130 and control is passed back to step 510. If no, control is passedonto step 550, where, if connected, the braking resistor 120 isdisconnected from the power bus 130.

With reference to the flow chart illustrated in FIG. 2E, an exemplaryrapid safety shutdown discharge method 180 of the ultracap pack 110 willnow be described. The safety discharge 180 assumes an embodiment wherethe engine coolant pump uses an engine power take off to circulate acooling fluid through the engine 250 and the liquid cooled brakingresistor 120. Other embodiments are possible that use different coolingloops and coolant pumps without deviating from the invention. At step600, a determination is made if the engine 250 is running. If no, thecontrol is passed onto step 610 to connect the generator 180 to thepower bus 130 and pass the control to step 620 where the generator 180spins the engine 250 with the fuel shut off to dissipate power. Power isdissipated and the coolant pump circulates coolant until the voltage ofthe power bus 130 drops below the minimum operating voltage of thegenerator 180. If the determination at step 600 is yes, the engine 250is running, control is passed to step 630 where the generator 180 isdisconnected from the power bus 130. Both steps 620 and 630 pass controlto step 640 where the braking resistor 120 is connected to the power bus130 and the discharge current from the ultracap pack 110 passes throughthe braking resistor 120. Thus, the ultracap pack 110 is dischargedtypically in less than a minute, compared to hours that are required todischarge the ultracap pack 110 passively. Without departing from thespirit of this invention, other embodiments may use only the brakingresistor 120 or only the engine/generator 250/180 or other combinationsof the braking resistor 120 and the engine/generator 250/180 to performa rapid discharge of the ultracap pack 110.

FIG. 3 is a circuit schematic diagram that shows an embodiment of theinvention that uses IGBT switching in a standard 3-phase inverter powerstage.

With reference to FIG. 3, an embodiment of the invention is shown wherethe hybrid-electric drive system uses a standard 3-phase IGBT powerinverter stage. T2 and T3 are always off. During precharge, T1 and T6are on and T4 and T5 are off. During acceleration discharging T1, T4,T5, and T6 are off. During braking regeneration and generator chargingof the ultracaps T1 and T4 are on, and T5 and T6 are off. Duringadditional brake resistor power dissipation during braking T4 and T5 areon, and T1 and T6 are off. T5 is the only one turned on to discharge theultracap pack through the braking resistor during a rapid safetydischarge.

With reference to FIG. 4, another embodiment uses the two extra IGBTswitching phases of a standard 8-phase inverter that uses 6 phases tocontrol two motors. T1 is always off. T2 is on during charging of theultracap pack from the generator or the regenerative braking and duringadditional brake resistor power dissipation during braking. T3 is on toconnect the braking resistor during braking or for discharging theultracap pack. T4 is on during precharge and otherwise off.

With reference to FIG. 5, four other embodiments are shown that usedifferent methods of connecting two phases of an inverter. Theseembodiments demonstrate the choices available within the invention todecouple the ultracap pack. FIG. 5A and FIG. 5C show the positive sideof the ultracap pack connected to the plus side of the inverter and theswitching is performed on the minus side, while FIG. 5B and FIG. 5D showthe minus side of the ultracap pack connected to the minus side of theinverter and the switching is performed on the plus side.

FIGS. 6, 7, and 8 are circuit schematic diagrams that show embodimentsof the invention that use the IGBT switching in four inverter phases.Because two Siemens Duo-Inverters are typically used in the ISE/SiemensELFA based hybrid-electric drive systems, the embodiments shown in FIGS.6, 7, and 8 offer more choices in a practical system for decoupling andisolation of components for safety and redundancy.

With reference to FIG. 6, the two embodiments shown use two phases ineach of a pair of inverters for the purpose of decoupling the twoinverters. FIG. 6A shows the minus sides of both inverters connectedtogether, but the plus sides of the inverters are isolated. FIG. 6A issimilar to FIGS. 5B and 5D where the minus side of the ultracap pack isconnected to the minus sides of the inverters, and the switching of theultracap pack and the braking resistor occurs between the plus side ofinverter 1 and the plus side of inverter 2.

FIG. 6B shows the plus sides of both inverters connected together, butthe minus sides of the inverters are isolated. FIG. 6B is similar toFIGS. 5A and 5C where the plus side of the ultracap pack is connected tothe plus sides of both inverters and the switching of the ultracap packand the braking resistor occurs between the minus side of inverter 1 andthe minus side of inverter 2.

The embodiments shown in FIGS. 3, 4, 5, 6, 7B, and 8A all have thebraking resistor connected to one side of the ultracap pack, but in theembodiments shown in FIGS. 7A, 8A, and 9 the braking resistor isdecoupled from the ultracap pack and only connected through a switch.

With reference to FIG. 7, the two embodiments shown use the extra twophases of a pair of inverters to accomplish a different decoupling oftwo inverters with the ultracap pack. FIG. 7A is similar to FIGS. 5B,5D, and 6A where the plus sides of the inverters are isolated, but theminus sides of both inverters are connected together and connect to theminus side of the ultracap pack. The switching of the ultracap packoccurs between the plus side of the ultracap pack and the plus side ofinverter 2. The braking resistor is connected to the plus side ofinverter 2 and is switched to the minus side of both inverters.

FIG. 7B is similar to FIGS. 5A, 5C, and 6B where the minus sides of theinverters are isolated, but the plus sides of both inverters areconnected together. The minus side of the ultracapacitor is connected tothe braking resistor and the minus side of inverter 2. From the minusside of inverter 2 the ultracap pack and the braking resistor,respectively, are switched to the plus side of inverter 2 and the plusside of inverter 1.

With reference to FIG. 8, the two embodiments shown use the two extraphases of a pair of inverters to obtain a different decoupling of twoinverters along with the ultracap pack.

FIG. 8A is similar to FIGS. 5A, 5C, 6B, and 7B where the minus sides ofthe inverters are isolated, but the plus sides of both inverters areconnected together and connect to the plus side of the ultracap pack.The braking resistor connects to the minus side of inverter 2 and isisolated from the ultracap pack. The ultracap pack switching occursbetween the minus side of the ultracap pack and the minus side ofinverter 2. The braking resistor switching occurs between the brakingresistor and the plus sides of both inverters.

FIG. 8B is similar to FIGS. 5B, 5D, 6A, and 7A where the plus sides ofthe inverters are isolated, but the plus sides of both inverters areconnected together. The plus side of the ultracapacitor is connected tothe braking resistor and the plus side of inverter 2. From the plus sideof inverter 2 the ultracap pack and the braking resistor, respectively,are switched to the minus side of inverter 2 and the minus side ofinverter 1.

With reference to FIG. 9, the circuit schematic shows the preferredembodiment of FIG. 8A imbedded into the connections of a hybrid-electricdrive vehicle that incorporates two traction motors, a generator, anultracapacitor pack, a braking resistor, a 3-phase AC electricaccessories motor for the hydraulic pump and air compressor, and a3-phase AC air conditioning heat exchanger compressor. The 150 kWgenerator and the traction motor M1 use three phases each of the eightphases available in inverter 1. Traction motor M2 and the AC powerrequire six phases of the eight phases of inverter 2. The three of theremaining four phases, two in inverter 1 and one in inverter 2,implement the switching for the ultracap pack and the braking resistor.During start up, the engine starts via a 24V starter. Inverter 1 bus ischarged to 500VD. IGBT's 1 and 3 close to initiate capacitor precharge.After the voltages have equalized, IGBT 1 opens and IGBT 4 connects theultracap. During acceleration, the vehicle accelerates on energy storagepower until the voltage reaches 500V, where the engine kicks in untilthe voltage is back up to 650VDC. If the voltage drops below 320VDC, thecapacitors disconnect via IGBT 3 and the vehicle operates on enginepower at 700VDC. During braking, the system controller reduces thevoltage to 320VDC and IGBT 3 closes. Braking starts immediately at acurrent of no more than 400 A. When the voltage approaches 700VDC, thebraking resistors are turned on via IGBT 2 to limit the voltage to700VDC.

In another aspect of the invention, the ultracap pack 110 is prechargedthrough braking regeneration (i.e., the ultracap pack 110 is prechargedthrough the braking resistors 120 using the energy from thedrive/traction motors 412 when they act like generators in the processof doing electric braking). In this case braking regeneration energy isused instead of energy from the engine/generator 250/180. This variationdoes not have the same switch connections to the high voltage DC bus asthe FIG. 9 embodiment(s) described above because each drive motor is ona different inverter, but for connections with both drive motors on thesame inverter, the source energy is from the drive/traction motors 412.The switching is performed through inverter switches shown and describedherein. This embodiment may be advantageous, for example, if the engine250 would not start with the starter, but the hybrid-electric vehiclecould be set rolling (e.g., downhill), then the braking regenerationcould store enough energy in the ultracap packs 110 to get the engine250 started from the generator 250 as is normally done during operationafter startup.

In a further aspect of the invention, the ultracap pack 110 isimmediately discharged through the braking resistor 120 for service andmaintenance safety, and as an end-of-the-day turn-off for quickerequalization in the ultracap pack 110 without having to depend on theparallel resistors (for equalization) within the ultracap pack 110. Theimmediate discharge may be accomplished through the inverter switchesshown and described herein. The braking resistor(s) 120, which areliquid cooled, release the discharged energy received from the ultracappack 110 in the form of heat.

FIGS. 10A and 10B are a combined electrical circuit schematic andalgorithm flow chart for an exemplary method of precharging/dischargingan ultracapacitor energy storage pack through a high-power brakingresistor.

It will be readily apparent to those skilled in the art that stillfurther changes and modifications in the actual concepts describedherein can readily be made without departing from the spirit and scopeof the invention as defined by the following claims.

1. A system for precharging an ultracapacitor energy storage cell pack,comprising: an ultracapacitor pack, a high power DC bus, anelectro-magnetic braking regeneration system, and a breaking resistor todissipate power from the electro-magnetic braking regeneration system;means for switching the braking resistor in series with theultracapacitor pack; means for switching the braking resistor in serieswith the ultracapacitor, with the high power DC bus; means fordetermining when to disconnect the braking resistor in series with theultracapacitor pack and connect the ultracapacitor pack directly withthe high power DC bus; means for disconnecting the braking resistor inseries with the ultracapacitor pack and connecting the ultracapacitorpack directly with the high power DC bus.
 2. The system of claim 1,wherein the high power DC bus is powered from a power source includingat least one of an engine/generator set, a fuel cell, a turbinegenerator set, a power grid, a propulsion motor in braking regenerationmode, and a battery pack.
 3. The system of claim 1, wherein the systemis a system of at least one of an electric vehicle and a hybrid-electricvehicle.
 4. The system of claim 1, wherein the determining meansincludes means for identifying an ultracapacitor pack in a state ofdischarge.
 5. The system of claim 1, wherein the determining meansincludes means for monitoring the ultacapacitor pack voltage.
 6. Thesystem of claim 1, wherein the determining means includes means formonitoring the power bus voltage.
 7. The system of claim 1, wherein thedetermining means includes means for determining when an ultracapacitorpack voltage and a power bus voltage are matched.
 8. The system of clam1, further including a timing system to determine when an ultracapacitorpack precharge is complete.
 9. The system of claim 1, wherein theswitching means include at least one of high power contactor relays andInsulated Gate Bipolar Transistors (IGBTs).
 10. The system of claim 1,wherein the switching means are liquid cooled.
 11. The system of claim1, further including means for increasing the voltage across the brakingresistor, to control the charge current, after the braking resistor isconnected in series with the ultracapacitor pack to shorten theprecharge time.
 12. The system of claim 2, wherein the power source, thebraking resistor, and the ultracapacitor pack are liquid cooled.
 13. Thesystem of claim 2, wherein the power source is a generator, and thegenerator is a permanent magnet generator.
 14. The system of claim 2,further including a power source input circuit with a power conditioninginverter, the power conditioning inverter being a DC to DC converter.15. The system of claim 14, wherein the inverter is a Pulse WidthModulated (PWM) inverter.
 16. The system of claim 14, wherein theinverter includes IGBT switching.
 17. The system of claim 1, furtherincluding the traction motor control circuit with a power controllinginverter.
 18. The system of claim 17, wherein the inverter is a PWMinverter.
 19. The system of claim 17, wherein the inverter includes IGBTswitching.
 20. The system of claim 17, further including tractionmotors, and the traction motors are at least one of AC induction motorsand permanent magnet motors.
 21. The system of claim 1, wherein theultracapacitor pack is a plurality of ultracapacitor packs connected inat least one of a series combination and a parallel combination.
 22. Thesystem of claim 1, wherein the braking resistor is a plurality ofresistors connected in at least one of a series combination and aparallel combination.
 23. The system of claim 1, wherein the brakingresistor is a heating element for liquids.
 24. The system of claim 1,wherein the ultracapacitor pack is any type of energy storage pack thatrequires a precharge function including a flywheel.
 25. The system ofclaim 1, further including means for determining the current charge rateof the ultracapacitor pack.
 26. The system of claim 25, wherein thedetermining means includes means for monitoring the voltage across thebraking resistor.
 27. A method of precharging an ultracapacitor energystorage cell pack, comprising: providing a drive system including anultracapacitor pack, a high power DC bus, an electro-magnetic brakingregeneration system, and a breaking resistor to dissipate power from theelectromagnetic braking regeneration system; switching the brakingresistor in series with the ultracapacitor pack; switching the brakingresistor in series with the ultracapacitor, with the high power DC bus;determining when to disconnect the braking resistor in series with theultracapacitor pack and connect the ultracapacitor pack directly withthe high power DC bus; disconnecting the braking resistor in series withthe ultracapacitor pack and connecting the ultracapacitor pack directlywith the high power DC bus.
 28. The method of claim 27, furtherincluding determining the ultracapacitor pack is in a state ofdischarge.
 29. The method of claim 27, further including monitoring theultracapacitor pack voltage.
 30. The method of claim 27, furtherincluding monitoring the power bus voltage.
 31. The method of claim 27,further including determining when the ultracapacitor pack voltage ismatched to the power bus voltage.
 32. The method of claim 27, furtherincluding using a timer to determine when an ultracapacitor precharge iscomplete.
 33. The method of claim 27, wherein the drive system includesat least one of software and firmware with control logic therein. 34.The method of claim 33, wherein the control logic is implanted by meansof a Programmable Logic Controller (PLC).
 35. The method of claim 27,further including reporting control commands and status reporting over aControl Area Network (CAN) data bus, the CAN data bus being a SAEStandard J1939.
 36. The method of claim 35, wherein the control commandsare part of the control strategy of a hybrid-electric vehicle drivesystem.
 37. The method of claim 27, wherein the drive is a hybridelectric vehicle drive system.